Combustion processes which burn fossil fuels introduce emissions into the atmosphere which have been linked with harmful effects. Environmental regulations have been enacted to limit the concentrations of these emissions in the exhaust gases from combustion processes. Such emissions include nitrogen oxides or NO.sub.x, primarily NO and NO.sub.2. Preferably, NO.sub.x emission levels should be significantly below 100 parts per million (ppm).
NO.sub.x emissions arise from nitrogen present in the combustion air and from fuel-bound nitrogen in coal or fuel oil if such fuels are burned. Conversion of fuel-bound nitrogen to NO.sub.x depends on the amount and reactivity of the nitrogen compounds in the fuel and the amount of oxygen in the combustion zone. Conversion of fuel-bound nitrogen is not present in processes using fuels such as natural gas, which contain no fixed nitrogen compounds.
Conversion of atmospheric nitrogen, N.sub.2, present in the combustion air to NO.sub.x (thermal NO.sub.x) is temperature dependent. In general, the greater the flame temperature in the combustion zone, the greater the resultant NO.sub.x content in the emissions. NO.sub.x conversion increases substantially at temperatures greater than 1800K if O.sub.2 is present.
Many industrial processes, such as forging, reheating, steel blooms, and melting of glass or aluminum, are carried out in high temperature, gas-fired furnaces. In such high temperature processes, air used in the combustion process is frequently preheated. Preheating the air reduces the amount of fuel needed, increasing thermal efficiency, but increases the temperature of the flame, which increases NO.sub.x content. Thus, a higher temperature burner which is capable of reducing NO.sub.x emissions without sacrificing thermal efficiency is needed.
One way of reducing NO.sub.x content which has been effective in processes using nitrogen bearing fuels is to create a fuel-rich combustion zone followed by a fuel-lean combustion zone. This can be achieved by staging the introduction of air into the combustion chamber. The fuel-rich zone contains less than the theoretical or stoichiometric amount of oxygen. Thus, less oxygen is available to convert the nitrogen to NO.sub.x.
Recirculating flue gas into the flame is another technique to limit NO.sub.x emissions. The recirculated flue gas reduces the oxygen concentration in the reactants and reduces the flame temperature by cooling the combustion products, thereby lowering NO.sub.x content. Additionally, NO.sub.x present in the recirculated flue gas can be further destroyed by reburning.
External ducting is typically provided from the furnace to the burner for recirculation of the flue gas. The flue gas is hot and accordingly heat is lost from the system as the gas passes along the ducting. The hot flue gases are sometimes used for other purposes, such as preheating the combustion air or vaporizing liquid fuels. This minimizes, but does not eliminate, the heat losses from the system.
Steel reheat furnaces, in which steel is heated to temperatures at which it can be plastically deformed by forging or rolling, are particularly suitable for the retrofitting of burners which utilize flue gas recirculation. Temperatures in these furnaces typically reach 1450K or greater, which makes the use of recuperators for preheating combustion air or regenerative burners cost effective. However, the external flue gas ducting of prior art burners, which incorporate flue gas recirculation, can render the retrofitting of such burners to an existing furnace difficult and costly. Moreover, the recirculation of cooled flue gas to the combustion zone reduces the thermal efficiency of the process.